The present invention is related to time division duplex (TDD) communication systems. More particularly, the present invention is directed to utilizing unused channel capacity during the transmission of a special burst (SB).
TDD communication systems utilize both common channels and dedicated channels. Once a dedicated channel has been established in a TDD system, when the transmitter does not have any user data to send, the transmitter enters a mode known as discontinuous transmission mode (DTX). DTX may exist in either the uplink or the downlink, and DTX may exist on the uplink independently from the downlink or on the downlink independently of the uplink.
When DTX is entered, the transmitter periodically sends a transmission known as a special burst (SB). Referring to FIG. 1, the SB comprises a midamble, two (2) transport format combination indicator (TFCI) fields, one transmit power control (TPC) field and two (2) data fields. The two (2) data fields are unused in an SB. Accordingly, arbitrary bits are used to fill in these data fields. These arbitrary bits are wasted channel capacity which would otherwise be transmitted and available for communicating information. Since these SBs are transmitted periodically to indicate DTX, a great deal of channel capacity is wasted. A normal burst (NB) is similar to an SB, except that the data fields contain useful data so the channel capacity is not wasted.
There are several reasons for transmitting SBs. The first reason is for fast channel establishment. Upon initial establishment of a physical channel, there is typically no data available. The transmitter must transmit some information to which the receiver can determine the availability of the physical channel. This permits the receiver to quickly recognize the availability of the channel so that data transmissions that require reliable Quality of Service (QoS) can be initiated.
The second reason for transmitting SBs occurs after a channel has been established and the channel enters DTX. The transmission of SBs avoids erroneous detection of “out-of-synchronization”. The out-of-sync condition results in termination of the physical channels which would otherwise occur if the receiver does not receive any transmission from the transmitter within a certain duration and with a specified quality. In essence, the out-of-sync condition releases the physical channel and permits the receiver to re-allocate unused physical channel resources. The periodic transmission of SBs avoids this condition when the physical channels are of acceptable quality by guaranteeing periodic transmission of a signal.
The receiver needs to be able to distinguish between a condition whereby no signal is received due to physical problems with the radio channel such as interference or obstructions, and when no signal is received because the transmitter simply does not currently have information ready for transmission. The use of SBs permits the receiver to distinguish between the transmitter entering DTX mode and the transmitter ceasing to communicate.
Since SBs do not send data that is utilized by the receiver, they are designed to be sent periodically in order to establish or continue the communication link. They are also designed to utilize minimum energy resources. However, the periodic transmission of SBs represents wasted channel capacity which may be utilized for exchange of information.
It would be desirable to have a system which does not waste this channel capacity.